Thack's right, any oil of roughly the right sae will do and certainly a small amount of the "wrong" oil will do no harm at all.
Mixing synthetic and mineral oil will also do no damage at all but you may, of course, loose the properties of the synth oil if you add too much mineral.
The only thing that was NOT ALLOWED and would quickly ruin an engine was mixing mineral and vegetable oil (castor based oils) but I don't suppose anyone uses them these days.
Shall we both duck now Thack?

Classic C5 Euro 4 at the moment.
Also CBf250.